The present invention relates to controlling temperatures in electronic components and especially to controlling the temperatures of electronic components in a tactical missile.
During the flight of a missile, waste heat is generated by the guidance and control systems. This heat must be dissipated. If the heat is not removed from the systems, they can overheat and fail. During supersonic flight, the outside surface of the missile is too hot to act as a radiator. Accordingly, the excess heat must be absorbed internally.
Flight time for tactical missiles is typically fairly short, on the order of five or six minutes at the most. During this time the electronics packages involved in controlling the flight generate a substantial amount of heat. This heat has been absorbed by appropriately sized metal heat sinks inside the missile. Typically a computer chip may have a copper or aluminum plate, with or without fins, fastened to it to store and re-radiate excess heat. Such heat sinks are able to keep the temperature of the electronics packages below unacceptable levels for the short time required for flight, although they add weight that does not directly increase performance.
The use of heat sinks for each thermally sensitive component ignores the heat capacity of other internal components of the missile such as the structural frame that holds the missile together and the propellant. A heat management system that uses the heat capacity of these internal components could reduce the size of or entirely eliminate many individual heat sinks within the missile.
Tactical missiles are also extensively bench tested and reprogrammed. This testing and reprogramming may take substantially longer than the actual flight time, especially where there are repeated simulations of combat situations. The heat sinks suitable for a six minute flight cannot keep the electronics packages cool enough for a lengthy test or reprogramming.
In the past the electronic components have been kept cool during testing and reprogramming by testing and programming briefly and then allowing the components to cool down. This has the disadvantage of prolonging testing and reprogramming times.
In another approach the components have been kept from overheating by making temporary mechanical connections between the internal heat sinks and the missile housing (skin) during testing. These mechanical connections have been made with thermal diodes that allow heat to flow from the heat sink to the housing so long as the housing is cooler than the heat sink. Such thermal diodes degrade missile performance by adding weight and expense.
Active cooling loops have also been used. These cooling loops provide internal cooling during testing and reprogramming by circulating a fluid heat transfer medium through passages inside the missile. While this allows cooling of the electronics during testing and reprogramming, the space occupied by the cooling system is wasted during tactical flight, thereby decreasing missile performance.
Sometimes specific hardware is created to cool the entire missile during testing and reprogramming. This is effective in the laboratory or at the factory, but usually the cooling equipment is not easily taken into the field for reprogramming during combat.
The present invention creates a thermal ground plane within a missile. The thermal ground plane connects all thermally significant components within the missile and keeps them at a uniform temperature. During the missile flight the ground plane absorbs excess heat keeping components cool and distributes heat quickly to heat absorbing components within the missile. During testing and reprogramming, the ground plane is attached to an external heat dissipation device through an opening in the skin of the missile. High flow rates of heat through the ground plane and its external cooling device maintain the electronics at a steady-state temperature below the unsafe operating temperature limit during testing and reprogramming.
The thermal ground plane is established within the missile using a heat pipe. This device relies on the circulation and phase change of a fluid to move heat from hotter regions to cooler regions. The heat pipe is connected to all the internal devices that need cooling and to any internal structure that can absorb heat. During tactical flight, the phase change of the fluid from liquid to gaseous and its re-condensation in cooler regions of the heat pipe where energy is absorbed provide enough thermal capacity to keep the components from over heating. Excess heat is rapidly transferred to structural, heat absorbing components of the missile. During testing the external cooling device is connected to the cool region of the heat pipe to draw excess heat out of the missile.
The invention improves missile performance since there are no wasted components carried during tactical flight and little wasted space. In addition, waste heat can be managed comprehensively rather than on a component by component basis.
A preferred embodiment uses a heat pipe to establish a thermal ground plane. Heat pipes have very high thermal conductivity, allowing heat to move rapidly. Like an electrical ground plane which has minimal resistance to the flow of electricity, a thermal ground exhibits minimal resistance to heat flow. For example, a heat pipe may have 10 times the thermal conductivity of a copper bus similarly configured. High thermal conductivity is an important feature of the present invention, and other devices or materials exhibiting high thermal conductivity could be used instead of the heat pipe. For example, encapsulated graphite fiber bundles could be used. The heat pipe may include branches which extend from it to absorb heat from high heat components. The branches may be made of metal such copper or may themselves be heat pipes.